1. Field of the Invention
This invention relates generally to a projector-type display device comprising a liquid crystal panel as light valve in which the light beam is modulated according to a video signal, and a lens through which the light beam passes and is projected on a screen in enlarged fashion. More specifically, this invention relates to a projector-type display device which prevents the light beam passing through the liquid crystal panel from reflecting off the lens or optical composite prism and back into the liquid crystal panel.
2. Description of the Related Art
Today, research in practical usage of a projector-type display device is underway, wherein a light beam from a light source is modulated according to a video signal as it passes through a translucent LCD panel and projected on a screen in enlarged fashion through a optical projector system. However, significant glare and optical feedback problems remain, as shown in the conventional projection display of FIG. 8. As shown in this figure, the portion of the light beam L which is reflected in the interface between a polarizing plate 6 and air (L.sub.R in FIG. 8) returns back to the liquid crystal panel to cause display and image washout errors. It is necessary to solve this problem as soon as possible, and a projector-type display device which has a .lambda./4 plate between the liquid crystal panel and the optical projector system is described in the Japanese Laid-open Patent Publication Hei 5-210097, as a known technique to reduce transmission of L.sub.R back to the LCD panel.
FIG. 9A shows the optical system of a conventional projector-type display device utilizing the aforementioned .lambda./4 polarizing plate. In FIG. 9A, the light beam emitted L from the light source lamp 10 passes through an optical modulating system 30, comprising an input-side polarizing plate 40, a liquid crystal panel 50, and an output-side polarizing plate 60. In the optical modulating system 30, the light beam impinged according to the video signal is modulated in the liquid crystal panel 50, then modulated again by passing through the output side polarizing plate 60 to form a linearly polarized light beam, and impinged into the .lambda./4 plate 70 which is disposed in the manner that the direction of its slow axis is off-aligned by 45.degree. with that of the light transmission axis of the output-side polarizing plate 60. Then, the light beam which passed the .lambda./4 plate 70 is projected on the screen 90 in enlarged fashion by the optical projector system 20.
On the other hand, the light beam which is reflected by the interface between output side of .lambda./4 plate and air passes through the .lambda./4 plate 70 again from the opposite side to result in a linearly polarized light beam whose polarizing direction is off-aligned by 90.degree. with that of the light transmission axis of the output-side polarizing plate 60. Therefore, the light beam which is reflected by the optical projector system 20 is absorbed by the output-side polarizing plate 60, not returning back to the liquid crystal panel.
FIG. 9B shows a configuration of optical system of a conventional projectortype display device wherein three (3) liquid crystal panels are applied. In FIG. 9B, the light beam emitted from the light source lamp 1 is separated into the light beams of red, green, and blue by means of a color separator system 100 which includes dichroic mirrors 101, 102. Then, the light beams separated by the color separator system 100 are impinged into the optical modulation system 30R, 30G, and 30B, which are respectively equipped with three (3) liquid crystal panels 50R, 50G, and 50B that correspond with the colors red, green, and blue as conducted by mirrors 111, 112, and 113. In each optical modulation system 30R, 30G, and 30B, the light beams of each color which is impinged according to the video signal are modulated by each liquid crystal panels 50R, 50G, and 50B, then modulated again by passing through the output-side polarizing plate 60R, 60G, and 60B to form linearly polarized light beams of the respective wavelengths. These linearly polarized light beams are then impinged into respective .lambda./4 plates 70R, 70G, and 70B, each disposed in the manner such that the direction of its slow axis is off-aligned 45.degree. with that of the light transmission axis of the output-side polarizing plate. Then the light beams which pass through the .lambda./4 plate 70R, 70G, and 70B are compounded by the light composite system 11 and are projected on the screen 90 in enlarged fashion by the optical projector system 20. On the other hand, the light beams which are reflected by the interface between output side of .lambda./4 plates 70R, 70G, 70B and air pass through the .lambda./4 plate 70R, 70G, and 70B again from the opposite side to result in the linearly polarized light beam whose polarizing direction is off-aligned by 90.degree. with that of the light transmission axis of the output-side polarizing plate 60R, 60G, and 60B. Therefore, the light beams which are reflected by the interface between output side of .lambda./4 plate 70R, 70G, and 70B and air are absorbed by the output-side polarizing plate 60R, 60G and 60B and do not return or reflect back to the liquid crystal panel.
With regard to these projector-type display devices, the light beams which pass through the optical modulation system 30R, 30G, and 30B (i.e. the light beams which are projected on the screen 90 in enlarged fashion), are modulated therein to form circularly polarized light beams because they must additionally pass through the .lambda./4 plate 70R, 70G or 70B prior to composition and projection. However, it is preferable that the light beam projected on the screen or the light impinged into the light composite system 11 is linearly polarized, as will be discussed hereinbelow.
Consider the case in which the light beam impinged in the light composite system is circularly polarized, as is described in connection with the conventional projection display devices illustrated in FIGS. 9A and 9B. Generally a dichroic prism having wave length selectivity is used for the light compositor system 11, and usually the dichroic prism specifies a wave length selectivity for exclusively either an S or a P-polarized component as is known in the art. This stems from the fact that while it is theoretically feasible to construct a dual selectivity dichroic prism, it is commercially impractical to produce such a device for the consumer projection market. Therefore, if the light composite system 11 described hereinabove is optimized for S-polarization selectivity, the S-polarized component of a red light beam emitted from the optical modulation system 30R, the light beam is reflected in the appropriate red reflecting film (not shown) of the light compositor 12 and conducted to the optical projector system 20. However, for the P-polarized component of the red light, a part of it passes through the red reflecting film. As a result, the red light beam is impinged into the optical modulation system 30B which opposes the optical modulation system 30R with the light composite system 11 interposing therebetween, resulting in perceptible glare, washout or partial disruption of the blue light beam. Moreover, similar problems exist when considering the P components produced by optical modulation systems 30G and 30R, or when the dichroic prism is optimized for P component selectivity.
Moreover, as ambient lighting normally projects from a ceiling-mounted device, reflected light of P-polarized direction tends to be more noticeable compared with that of the S-polarized direction. Therefore, a polarization screen fitted to the projection device which absorbs the P-polarized light component would normally be employed. However, if the light beam projected on the screen is circularly polarized, this-polarization screen would filter out P-polarized ambient light and the P-polarized component of the projected light beam. Thus, the intensity of the light beam projected on the screen would be cut in half and overall brightness of the projected image be diminished. Therefore, it would preferable that the light beam projected on the screen or the light impinged into the light composite system is linearly polarized so that only the P-polarized ambient light be filtered out by this polarization screen.